1. Field of the Invention
The present invention relates to a high-frequency power amplifier, and more particularly to a high-frequency power amplifier for use in mobile communication equipment including mobile telephones.
2. Description of the Background Art
In recent years, mobile telephones have become indispensable to our lives not only as calling tools but also as communication tools for exchanging E-mails, content, etc. Currently, the most prevalent mobile telephone communication systems used in the world are the second generation communication systems. In Japan, the personal digital cellular (PDC) system is widely used; in North America, the code division multiple access (CDMA) system is widely used; in Europe, the global system for mobile (GSM) system is widely used.
It is expected that in the near future, the market for mobile telephones will experience a transition to the third communication systems, such as the wideband code division multiple access (W-CDMA) system and the universal mobile telecommunications (UMT) system, enabling more reliable and faster communication of large-volume content data such as video images. In order to enable adaptation to such a change of the market, compact and multifunctional mobile telephones compatible with the third-generation communication systems and base station equipment therefor are currently under development.
Generally, in the market for mobile telephones, there have been needs for more compact and lighter mobile telephones having longer talk and standby times. In order to realize such mobile telephones, it is necessary to reduce the size of batteries used for the mobile telephones and to achieve higher efficiency (higher power savings) of power-consuming transmission power amplifiers.
The transmission power amplifiers for use in mobile telephones are called power amplifier (PA) modules. Conventionally, the PA modules include GaAs transistors having satisfactory high-frequency characteristics and a satisfactory conversion efficiency in converting direct current into a high-frequency signal. The GaAs transistors are generally classified into field-effect transistors (hereinafter, abbreviated as “FETs”), and heterojunction bipolar transistors (hereinafter, abbreviated as “HBTs”).
As an exemplary PA module for use in the mobile telephones, Japanese Laid-Open Patent Publication No. 11-195932 discloses a PA module which includes GaAs FETs. However, the PA module including the GaAs FETs requires a power supply for applying negative bias voltage to a gate electrode, resulting in an increase in number of components of the PA module, and making it difficult to reduce the size, weight, and cost of the mobile telephones. Thus, recent years have seen the GaAs FETs replaced with the GaAs HBTs, as transistors for use in the PA modules, which do not require any negative power supply and can operate on a single positive power supply.
Hereinbelow, a conventional PA module using GaAs HBTs for two-stage power amplification is described with reference to the drawings. FIG. 9 is a circuit diagram of the conventional PA module. The PA module shown in FIG. 9 includes high-frequency signal amplification HBTs 101 and 102, an input terminal Pin, an output terminal Pout, earth terminals, collector power supply terminals Vcc1 and Vcc2, choke coils 108 and 109, an upstream stage bias circuit 107, a downstream stage bias circuit 110, an input matching circuit 105, an interstage matching circuit 140, and an output matching circuit 106.
A high-frequency signal is inputted from the input terminal Pin through the input matching circuit 105 to the high-frequency signal amplification HBT 101. The input matching circuit 105 performs impedance matching for the high-frequency signal inputted from the input terminal Pin, thereby preventing the high-frequency signal from being reflected by the high-frequency signal amplification HBT 101. The high-frequency signal is inputted to the high-frequency signal amplification HBT 101, and amplified therein. Thereafter, the amplified signal is transferred through the interstage matching circuit 140 to the high-frequency signal amplification HBT 102.
The interstage matching circuit 140 performs impedance matching on the high-frequency signal amplified by the high-frequency signal amplification HBT 101, thereby preventing the high-frequency signal from being reflected by the high-frequency signal amplification HBT 102. The high-frequency signal is inputted to the high-frequency signal amplification HBT 102 and amplified therein. Thereafter, the amplified signal is outputted from the output terminal Pout via the output matching circuit 106. The output matching circuit 106 performs impedance matching on the high-frequency signal amplified by the high-frequency signal amplification HBT 102, thereby preventing the high-frequency signal from being reflected from outside the PA module.
The high-frequency signal amplification HBT 101 has a collector electrode connected to the collector power supply terminal Vcc1 via the choke coil 108. The choke coil 108 prevents the high-frequency signal outputted by the high-frequency signal amplification HBT 101 from leaking out from the collector power supply terminal Vcc1. Also, the high-frequency signal amplification HBT 101 has abase electrode connected to the upstream stage bias circuit 107. The upstream stage bias circuit 107 supplies bias current to the base electrode of the high-frequency signal amplification HBT 101.
Similarly, the high-frequency signal amplification HBT 102 has a collector electrode connected to the collector power supply terminal Vcc2 via the choke coil 109. Also, the high-frequency signal amplification HBT 102 has a base electrode connected to the downstream stage bias circuit 110. The downstream stage bias circuit 110 supplies bias voltage to the base electrode of the high-frequency signal amplification HBT 102.
The upstream stage bias circuit 107 and the downstream stage bias circuit 110 have a function of temperature-compensating the high-frequency signal amplification HBTs 101 and 102, respectively. These bias circuits are designed such that constant idling current flows to the high-frequency signal amplification HBTs 101 and 102 regardless of ambient temperature.
Hereinafter, the structure of the upstream stage bias circuit 107 is described with reference to FIG. 10. FIG. 10 is a circuit diagram of a high-frequency power amplifier used as an upstream stage amplification section of the conventional PA module. The high-frequency power amplifier shown in FIG. 10 includes the high-frequency signal amplification HBT 101, the input terminal Pin, an earth terminal, the collector power supply terminal Vcc1, the choke coil 108, the upstream stage bias circuit 107, the input matching circuit 105, the interstage matching circuit 140, and a resistor 115.
The upstream stage bias circuit 107 includes a reference voltage terminal Vref, resistors 111 and 114, earth terminals, a temperature compensation circuit 112, and a direct current supply HBT 113. The resistor 111 and the temperature compensation circuit 112 are connected between the reference voltage terminal Vref and an earth terminal. Node A shown in FIG. 10 is a point at which the resistor 111 and the temperature compensation circuit 112 are connected to each other. Potential at the node A is controlled by changing a resistance value of the resistor 111.
The direct current supply HBT 113 has a base electrode connected to the node A. The direct current supply HBT 113 has an emitter electrode grounded via the resistor 114. The emitter electrode of the direct current supply HBT 113 is also connected to the base electrode of the high-frequency signal amplification HBT 101 via the resistor 115. With the above structure, the direct current supply HBT 113 supplies bias current to the high-frequency signal amplification HBT 101.
The temperature compensation circuit 112 includes temperature compensation HBTs 112a and 112b. The temperature compensation HBT 112a has base and collector electrodes connected to the node A. Also, the temperature compensation HBT 112a has an emitter electrode connected to base and collector electrodes of the temperature compensation HBT 112b. The temperature compensation HBT 112b has an emitter electrode which is grounded. The temperature compensation HBTs 112a and 112b are transistors having the same characteristics as those of the direct current supply HBT 113 and the high-frequency signal amplification HBT 101. With the above structure, the temperature compensation circuit 112 temperature-compensates the high-frequency signal amplification HBT 101.
The reception efficiency of the mobile telephone is defined by the receiver sensitivity. If noise in a reception band (Rx noise) outputted from the PA module is greater than a predetermined value, the receiver sensitivity of the mobile telephone is reduced, adversely affecting the reception efficiency. A tolerable Rx noise level is generally less than or equal to −137 dBm/Hz.
In mobile telephones in which conventional GaAs FETs are used in the PA modules, the Rx noise level is less than or equal to −140 dBm/Hz, causing no adverse effect on the use of the mobile telephones. However, in recent years, the GaAs FETs used in the PA modules have been replaced by the GaAs HBTs, attracting attention on adverse effects of the RX noise on the receiver sensitivity of mobile telephones.
The conventional PA module shown in FIG. 9 was actually produced with GaAs HBTs designed to have a maximum oscillation frequency of 25 GHz, and the Rx noise of the PA module was measured. In the case where the PA module was set so as to have output power of 28 dBm and gain of 28 dB in a transmission band from 824 MHz to 849 MHz, the Rx noise level in a reception band from 884 MHz to 909 MHz was −132 dBm/Hz. As a result, it was found that in the conventional PA module using the GaAs HBTs, the Rx noise is at such a level as to adversely affect the receiver sensitivity of the mobile telephones.
Typically, the Rx noise level of a PA module can be calculated based on a thermal noise level (=−174 dBm/Hz), gain (Grx) in the reception band of the PA module, and a noise factor (NF) of the PA module. In a conventional PA module using the GaAs FETs, if Grx=28 dB and NF=4 dB, the Rx noise level is −142 dBm/Hz. This result is approximately consistent with measured values.
On the other hand, in a conventional PA module using the GaAs HBTs, if Grx=28 dB and NF=6 dB, the Rx noise level is −140 dBm/Hz. Accordingly, it is understood that the above measured value (Rx=−132 dBm/Hz) considerably exceeds the theoretical value.
Upon analysis of differences between the PA module using the GaAs HBTs and the PA module using the GaAs FETs based on the experimental result of the Rx noise level of the PA module using the GaAs HBTs, and the result of the theoretical calculation, it can be said that the source of the Rx noise in the PA module using the GaAs HBTs is the bias circuit. Specifically, it can be said that the Rx noise of the PA module using the GaAs HBTs occurs because 60 MHz noise corresponding to a difference in frequency between transmission and reception bands (hereinafter, referred to as a “difference frequency signal”) is generated in the upstream stage bias circuit 107, and the difference frequency signal and a transmission wave signal are mixed together in the PA module.
Also, recent years have seen active reduction in size of duplexers for realizing more compact mobile telephones. As compared to before, the duplexers are becoming unsatisfactory in terms of attenuation levels in the reception band. In particular for the PA module using the GaAs HBTs, which generally has a higher Rx level than the PA module using the GaAs FETs, the above problem related to the Rx noise requires urgent resolution.